In conventional radio communication systems, signals are typically transmitted between a base station (such as a radio transmission tower) and a mobile station receiver (such as a mobile telephone). The signals received by the mobile station receiver often differ from the original transmitted signal due to interference from the physical environment during signal propagation. By the time the transmitted signal is received by the mobile station receiver, the signals from different propagation paths may be associated with different phase delays.
Some conventional communication systems, such as Third Generation Partnership Project (3GPP) Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access-Frequency Division Duplex (UTRA-FDD) systems, operate using a pilot channel. The pilot channel represents a continuous modulation of a known bit sequence of a transmitted signal. Using the pilot channel, the mobile station receiver may estimate the phase of different received signals corresponding to different propagation paths before the mobile station receiver combines the different signals together.
Code Division Multiple Access (CDMA) systems are spread spectrum systems that employ radio channels with a much larger bandwidth compared to conventional frequency modulated (FM) systems. For example, a bandwidth of 1.5 MHz may be used in Interim Standard 95 (IS-95) systems, and a bandwidth of 5.0 MHz may be used in 3GPP UTRA-FDD systems. In CDMA systems, a single radio channel can simultaneously support multiple users, as opposed to FM systems that can only support one user per radio channel.
In order to differentiate information intended for different recipients transmitted over a shared radio channel in a CDMA system, each user is typically assigned a unique pseudo-noise (PN) code. Each bit in an information sequence, such as a stream of bits, to be transmitted to a particular user on a shared channel is typically multiplied with the intended recipient's PN code. The PN code is a stream of chips, where each bit of the information sequence correlates to spreading factor chips of the PN code. This technique is known as spreading and is one of many ways of transmitting a signal intended for one user over a shared channel. The signal is received by all users of the shared channel but can only be successfully decoded by the mobile station receiver containing the intended recipient's PN code. This technique of encoding data using a PN sequence is also applicable in the case of uplink communications, where a mobile station receiver sends data to a base station.
A mobile station receiver that has knowledge of a particular user's PN code is able to differentiate information sequences that are intended for that particular user from other information sequences intended for other users on the shared channel. Bits in the original transmitted information sequence are extracted from a received signal by de-correlating the PN code with the signal received by the mobile station receiver. This process of correlation is known as de-spreading.
For CDMA systems to work properly, each of the following usually needs to occur. The signals received by the mobile station receiver and the PN code used for de-spreading need to be time-aligned. Also, the auto-correlation properties of the PN codes are preferably high, meaning the correlation is high if a sequence is correlated with a zero-shifted sequence of itself and nearly zero if a sequence is correlated with a non-zero time-shifted sequence of itself. In addition, the cross-correlation properties of the PN codes are preferably near ideal, meaning nearly zero.
As such, CDMA systems may employ a pilot signal to maintain the integrity of the above conditions. The pilot signal in a CDMA system is typically a constant bit sequence that is spread using a predefined PN code, which is specific to a particular base station. Both the pilot signal bit sequence and the PN code used for its spreading remain the same for the particular base station. The pilot signal is spread and transmitted by the base station over the shared channel.
FIG. 1 illustrates a conventional rake receiver 10 in a mobile station receiver. The rake receiver 10 includes a plurality of rake fingers 12. A received input signal 11 can be considered as the sum of many multipath signals. The received input signal 11 is passed along a plurality of processing paths via the rake fingers 12 in the mobile station receiver. Each rake finger 12 is assigned to de-spread one of the many multipath signals from the received input signal 11. The output of each rake finger 12 is sent to a combiner 15. The combiner 15 combines all de-spread outputs from the rake fingers 12 and sends a combined output signal 17 for further processing. This type of rake receiver 10 may also be used in a base station or other device and is not limited to use in a mobile station receiver.
FIG. 2 illustrates a conventional rake finger 12 in the rake receiver 10 of FIG. 1. The rake finger 12 receives the input signal 11, which includes traffic signals (bits corresponding to data symbols) and pilot signals (bits corresponding to pilot symbols). In this example, the rake finger 12 includes a traffic PN code generator 22, which generates a particular unique traffic PN code. A traffic correlator 24 correlates the traffic PN code generated by the traffic PN code generator 22 with the received input signal 11. The traffic correlator 24 outputs de-spread data symbols 27. Similarly, a pilot PN code generator 21 and a pilot correlator 23 operate to produce de-spread pilot symbols 18. The number of inputs (also known as “chips”) from the input signal 11 used to de-spread one output bit corresponding to a data symbol 27 or a pilot symbol 18 is known as the spreading factor (SF). The spreading factor for the data and pilot symbols can be the same or different. For the data symbols 27, permissible spreading factor values are typically 4, 8, 16, 32, 64, 128, and 256. For the pilot symbols 18, the permissible spreading factor value is typically fixed at 256.
Due to the time varying nature of a shared channel, the data and pilot symbols often undergo attenuation and phase rotation (or phase delay) by the time the symbols reach the mobile station receiver or the base station. Estimates of the attenuation and phase delay are determined in an adaptive channel estimation unit 29. For this purpose, the pilot correlator 23 outputs de-spread pilot symbols 18 to the adaptive channel estimation unit 29, and the adaptive channel estimation unit 29 outputs channel estimates 28 to a phase correction unit 25. The phase of the received signal 11 is corrected by the phase correction unit 25. The phase correction unit 25 determines the phase delay for the current portion of the received input signal 11 using the channel estimates 28 from the channel estimation unit 29. The phase correction unit 25 accordingly corrects any phase delay in the current portion of the de-spread data symbols 27 from the traffic correlator 24.
To achieve optimal performance in a wideband CDMA (W-CDMA) system, it is often important that the pilot and traffic PN codes are synchronized to the signal received by the mobile station receiver (i.e. they are kept time-aligned). A timing and control unit 26 corrects the timing and helps maintain the time alignment between the PN code generators 21-22 and the received signal 11.
The channel estimates 28 in the rake receiver 10 are often corrupted by additive white Gaussian noise (AWGN) and fading. Averaging the AWGN results in the reduction of AWGN noise power. Typically, larger averaging lengths result in better noise reduction. However, in a fading channel, if the averaging length is too long, the dynamics of the channel change and the results will deteriorate. Therefore, shorter averaging lengths typically result in better fading immunity, provided there is no deep fade occurring at that instance. A deep fade is a condition where the signal quality is too bad for any estimation purposes. Due to these two contradicting effects (AWGN and channel dynamics), the accuracy of the channel estimates 28 and a bit error rate (BER) achieved are dependent on the averaging length employed.
The optimum averaging length to obtain the channel estimates 28 is often dependent on the speed of the mobile station receiver and the signal-to-noise ratio (SNR) of the received signal 11. It therefore often becomes necessary to identify the optimum averaging length to improve the channel estimates 28 as the speed of the mobile station receiver varies.